Tesla High Voltage Battery Pack Information

Common Failure Modes

Here are the Big-3 most common reasons a pack fails

v1 - C26/C27 BMB Failure

Discussed in more detail In This Section, the v1 packs have a fatal (but correctable) flaw. The plastic shield rub two capacitors on the voltage sensing circuit and cause them to corrode. When the BMB can't accurately calculate the voltage in each brick, all sorts of crazy things can happen. The BMS can interpret this with one or a combination of 018, 029 or 107 fault codes plus several others. 018 will clear itself after repair (often needs to sit overnight) but 107 and 029 latch and will need to be cleared with Toolbox.

v2 - Sense Wire Failure

A good discussion here with a lot of details from a member going by AlphaTango11. In brief, the little sense wires break off the copper pad - especially off brick #3 which is upside down and the longest of the 'flexible' ribbon cables so this piece is essentially hanging in the air instead of laying on the top of the module. This appears to be very common in packs that have been involved in accidents and subjected to unusual impact G loads. Some are speculating thermal expansion is to blame - the module and flex cable expand and contract at different rates and these tiny welds fatigue and break. Certainly both of these can contribute to the problem together. Like any of these repairs, it is possible, but you have to remove and open the pack first. Best practice repair techniques are still being worked out.

Broken sense wires throw the BMS_f107 error and need a Toolbox to reset after the physical repair is done.

Rusty Fuse Cover - V1

The V1 packs have a metal fuse cover that faces up. In rustbelt areas, thousands of packs have died when this cover rusts through and liquid water gets into the pack. This usually results in the internal isolation error BMS_f123. When wanter gets in the pack, time is of the essence. Left unattended, the wet bottom plates begin to corrode and rust which can quickly leave the pack unrepairable. The two failure modes above are relatively easy to repair, but once water gets into your pack, you are on borrowed time. While corroded BMBs and broken sense wires are something to repair after the pack fails (not preemptively), every owner of a v1 pack with a top-facing fuse cover should have it inspected for rust regularly. You can see most of it through the frunk on a rear wheel drive car but dual motor cars are more crowded around there so the pack needs to come out for a thorough inspection. Do a Pressure Test and Reset The Umbrella Valves while you have it out. Many, many emergency repairs can be avoided if you catch this early and keep the pack dry.

Environmental Moisture Ingress

The corrosion on C26 and C27 is caused by the contact from the plastic shield but that contact is made worse by moisture in the pack. The vast majority of failures are near the front of the pack where the pack breathes. The moisture that is present condenses across the contact point between the plastic shield and C26/C27 so the plastic shield is the problem but the higher relative moisture presence at the front of the pack is a contributing factor. A good post on the topic: https://teslamotorsclub.com/tmc/posts/8403605/

Resistive Cells

There are some famous videos where celebrity hosts talk about how easy it is to 'clip out the bad cell' and make the pack live on for many more miles. I personally believe actual cell failures are rare. How many 'bad modules' in the v1 packs were just failed BMBs that have caused the voltage readings to go haywire and caused the bleed resistors to make actual (correctable) imbalances? Remember, Tesla doesn't 'fix' or diagnose anything at the Service Center. If your pack came in with one of these problems, they will offer to install a refurbished pack and send your pack off to fix whatever acute issue caused it to die in a contracted 3rd party rehab facility. Your 'repaired' pack would go out to be installed into the next person's car. Until 2020, there were no Tesla Model S out of warranty (all cars in that range were 8 years/unlimited miles) so nobody really started trying to figure these things out until relatively recently. I'm sure it took a while for the early 3rd party repair shops to fully understand some of these issues and we are still learning more every day. The Big-3 listed here cover the vast majority of issues you will encounter with v1 and v2 packs. What will we learn in the future?

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